Isotopic fractionation and zero-point effects in anionic H-bonded complexes: a comparison of the I− · HDO and F− · HDO ion–molecule clusters

Mid-infrared spectra of the argon-solvated X− · H2O, X− · HDO, and X− · D2O complexes (X=I, F) are reported in both the OH and OD stretching regions. Due to the asymmetric H-bonding arrangement adopted by the monohydrates, two isomers are possible for the mixed isotopomer (X− · DOH and X− · HOD). The spectra establish that the isomeric form with ionic H-bonding to the OD group is preferentially formed for the iodide complex (I− · DOH), while the fluoride ion binds preferentially to OH (F− · HOD). The behavior of the iodide monohydrate is traced to the zero-point contribution from the ion–molecule, out-of-plane bending vibrational mode, while the very strongly red-shifted, anharmonic OH stretches appear to overcome this effect in the F− · HOD complex.